Using artificial signals to maximize capacity and secrecy of multiple-input multiple-output (mimo) communication

ABSTRACT

A system and method for increasing the capacity of a Multiple-Input Multiple-Output (MIMO) system at desired user&#39;s locations and reducing the capacity at locations, other than that of the desired user, while also providing secrecy. Knowing the channel coefficient between each transmitter and receiver antenna pair at the transmitter, the method of the present invention calculates the artificial signal that minimizes the Euclidean distance between the desired and received data symbols if the precoding/combining matrix pair from the set that has the minimum Euclidean distance to the singular value decomposition (SVD) of the channel matrix is used for transmission and reception. The artificial signal may be fed to the precoder, instead of the actual desired data symbols, or may be transmitted directly to reduce computational complexity, power consumption and processing delay if the hardware configuration allows.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of U.S. patentapplication Ser. No. 16/239,372 filed on Jan. 3, 2019, entitled “ UsingArtificial Signals to Maximize Capacity and Secrecy of Multiple-InMultiple-Output (MIMO) Communication”, which claims priority to U.S.Provisional Patent Application No. 62/682,421 filed on Jun. 8, 2018,entitled “Using Artificial Noise to Maximize Capacity and Secrecy ofMIMO Transmitters that Use Analog/Hybrid/Codebook Based DigitalPrecoders”, both of which are incorporated by reference herein in theirentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No. 1609581awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

In a theoretical, ideal, Multiple-Input Multiple-Output (MIMO) antennasystem, comprising multiple transmitting antenna at a transmitter andmultiple receiving antenna at a receiver, after learning the channelmatrix H between the transmitter and the receiver, the transmitterdecomposes the channel matrix to its singular values:

H=UΛV*

Where the ith row and jth column of H contains the flat fading channelcoefficient between the ith receiver and jth transmitter antenna, and Ais a diagonal matrix containing the singular values of H on its maindiagonal. Using this decomposition, the transmitter precodes theQuadrature Amplitude Modulation (QAM symbols), {tilde over (x)} by leftmultiplication with the V matrix to obtain the transmitted signal x.Namely, x=V{tilde over (x)} is the signal fed to the antennas whichpasses through the transmission channel.

After receiving the transmitted signal, the receiver performs the postprocessing by left multiplication with U*, effectively creating:

{tilde over (y)}={tilde over ({circumflex over (x)})}=U*HV{tilde over(x)}

{tilde over (y)}={tilde over ({circumflex over (x)})}=U*UΛV*V{tilde over(x)}

{tilde over (y)}={tilde over ({circumflex over (x)})}=Λ{tilde over (x)}

Wherein, {tilde over (x)} is the data symbols, V is the pre-processingmatrix obtained from the singular value decomposition (SVD) of thechannel matrix, x is the vector of signals fed to the transmitterantenna, y is the column of observations at the receiver antenna, U* isthe post-processing matrix based on the channel and {tilde over (y)} isthe estimated data symbols that are obtained by post-processing theobservations ({tilde over (y)}=Λ{tilde over (x)}).

If analog, codebook based digital or hybrid beamforming is used in aMIMO system, the pre-processing matrix V and the post-processing matrixU* can only be chosen from a set of pre-determined matrix pairs,hereinafter referred to as the matrix dictionary. Since the matrix pairsin the dictionary do not match the channel counterparts, the channelcannot be decomposed to its singular values completely and there aremismatches between the precoder/combiner and the actual transmissionchannel. This mismatch reduces the capacity of the MIMO channel.Furthermore, since only a finite set of known precoder/combiner matricesare used, an eavesdropper can obtain the transmitted signal correctly bybrute-force searching all combiners in the set.

Accordingly, what is needed in the art is an improved system and methodthat increases the capacity of a MIMO transmission channel, while alsoproviding for secrecy of the communication over the channel.

The present invention may address one or more of the problems anddeficiencies of the prior art discussed above. However, it iscontemplated that the invention may prove useful in addressing otherproblems and deficiencies in a number of technical areas. Therefore, theclaimed invention should not necessarily be construed as limited toaddressing any of the particular problems or deficiencies discussedherein.

All referenced publications are incorporated herein by reference intheir entirety. Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

While certain aspects of conventional technologies have been discussedto facilitate disclosure of the invention, Applicants in no way disclaimthese technical aspects, and it is contemplated that the claimedinvention may encompass one or more of the conventional technicalaspects discussed herein.

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date, publicly available, known to thepublic, part of common general knowledge, or otherwise constitutes priorart under the applicable statutory provisions; or is known to berelevant to an attempt to solve any problem with which thisspecification is concerned.

SUMMARY OF THE INVENTION

In various embodiments, the present invention provides a system andmethod that utilizes artificial signals to maximize capacity and secrecyof MIMO transmitters that utilize digital beamforming to communicate inan analog/codebook based digital/hybrid MIMO communication system. Inaccordance with the present invention, the transmitted signal isgenerated using a convex optimization that minimizes the effects of themismatch between the multiple antenna communication channels and thequantized precoding and combining operations. In this context,quantization refers to quantization of an infinite number of possiblechannel matrices to a finite set of precoding/combining matrices, and isnot related to quantization of analog signals, as used widely in theelectrical engineering literature.

In one embodiment, the present invention provides a codebook-basedmultiple-input multiple-output (MIMO) transmission method. The methodincludes, selecting a precoding/combining matrix pair, wherein theprecoding/combining matrix pair is selected based upon an estimatedchannel coefficient of a transmission channel between a MIMO transmitterand a MIMO receiver. The method further includes, generating anartificial signal from an information signal to be transmitted by theMIMO transmitter, wherein the artificial signal minimizes an errorbetween the information signal and the signal recovered by the MIMOreceiver following the application of a combining operation based uponthe precoding/combining matrix pair upon reception from the wirelesschannel.

In the present invention, the estimated channel coefficient of thetransmission channel is a matrix comprising an estimated channelcoefficient between each transmitter and receiver antenna pair at theMIMO transmitter and either the MIMO transmitter or the MIMO receiverestimates the channel coefficient of the transmission channel betweeneach one of a plurality of pairs of MIMO transmitting and receivingantennas and then selects the precoding/combining matrix pair thatmaximizes a capacity of the transmission channel based upon theestimated channel coefficient and notifies the communication counterpartof this choice.

Additionally, the artificial signal is generated by performing convexoptimization, wherein the artificial signal is designed to match desireddata symbols as much as possible upon transmitting the plurality ofartificial signals and after applying the combining matrix to theplurality of received artificial signals at the desired receiver, whilebeing limited by a power limitation.

In an additional embodiment, the present invention provides a codebookbased multiple input multiple output (MIMO) transmitter for increasingthe capacity of the (MIMO) system and for providing secrecy. The MIMOtransmitter includes, a signal processing unit for receiving aprecoding/combining matrix pair identifier, wherein theprecoding/combining matrix pair identifier is based upon an estimatedchannel coefficient of a transmission channel. The signal processingunit is further for generating an artificial signal from an informationsignal to be transmitted by the MIMO transmitter, wherein the artificialsignal minimizes an error between the information signal and the signalrecovered by the MIMO receiver following the application of a combiningoperation based upon the precoding/combining matrix pair upon receptionfrom the wireless channel.

In this embodiment, the estimated channel coefficient of thetransmission channel is a matrix comprising an estimated channelcoefficient between each transmitter and receiver antenna pair at theMIMO transmitter. In an exemplary embodiment, the signal processing unitmay be a modem.

In another embodiment, the present invention provides one or morenon-transitory computer-readable media having computer-executableinstructions for performing a method of running a software program on acomputing device, the computing device operating under an operatingsystem. When executed at an MIMO transmitter, the instructions from thesoftware program include, receiving a precoding/combining matrix pairidentifier from a MIMO receiver, wherein the precoding/combining matrixpair identifier is based upon an estimated channel coefficient of atransmission channel, generating an artificial signal from aninformation signal to be transmitted by the MIMO transmitter, whereinthe artificial signal minimizes an error between the information signaland the signal recovered by the MIMO receiver following the applicationof a combining operation based upon the precoding/combining matrix pairupon reception from the wireless channel.

In the present invention, the implemented algorithm is backwardcompatible with legacy standards and receivers and the modifications areperformed exclusively at the transmitting device, which transmits asignal that is designed to be received by legacy devices.

In addition, enhanced performance of the MIMO system can be realizedmodifying only the software, or the signal processing unit (modem), atthe transmitting device to utilize the introduced algorithm withoutrequiring modification of the receiving hardware.

By employing the method of the present invention at a MIMO transmitter,the spectral efficiency of the transmission channel is increased,thereby allowing faster data rates, increased connectivity and lowerenergy consumption. Additionally, the secrecy of the communicationchannel increases, as the transmitted signal is tailored to thetransmission channel of the intended receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1A is a block diagram illustrating the operation of a MIMO radioantenna system which includes precoding of the artificial signal, inaccordance with an embodiment of the present invention.

FIG. 1B is a block diagram illustrating the operation of a MIMO radioantenna system, without precoding of the artificial signal, inaccordance with an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a pair of MIMO devicesimplementing the artificial signal scheme, in accordance with anembodiment of the present invention.

FIG. 3 is a flow diagram illustrating the methods steps for artificialsignal transmission in a codebook-based MIMO system, where theprecoding/combining matrix pair is selected by the MIMO transmitter andno precoding of the artificial signal is performed, in accordance withan embodiment of the present invention.

FIG. 4 is a flow diagram illustrating the methods steps for artificialsignal transmission in a codebook-based MIMO system, where theprecoding/combining matrix pair is selected by the MIMO transmitter andprecoding of the artificial signal is performed, in accordance with anembodiment of the present invention.

FIG. 5 is a flow diagram illustrating the methods steps for artificialsignal transmission in a codebook-based MIMO system, where theprecoding/combining matrix pair is selected by the MIMO receiver and noprecoding of the artificial signal is performed, in accordance with anembodiment of the present invention.

FIG. 6 is a flow diagram illustrating the methods steps for artificialsignal transmission in a codebook-based MIMO system, where theprecoding/combining matrix pair is selected by the MIMO receiver andprecoding of the artificial signal is performed, in accordance with anembodiment of the present invention.

FIG. 7 is a graphical illustration comparing the received errormagnitude of precoded and nonprecoded artificial signal transmission forcodebook-based MIMO systems to other solutions known in the art as afunction of correlation between the precoder/combiner and wirelesstransmission channels for infinite signal to noise ratio (SNR).

FIG. 8 is a graphical illustration comparing the secrecy capacity ofprecoded and nonprecoded artificial signal transmission forcodebook-based MIMO systems to other solutions known in the art as afunction of the correlation between the precoder/combiner and wirelesstransmission channels for infinite SNR.

FIG. 9 is a graphical illustration comparing the bit error rate (BER) ofprecoded and nonprecoded artificial signal transmission forcodebook-based MIMO systems to other solutions known in the art as afunction of the correlation between the precoder/combiner and wirelesstransmission channels for 3 decibel SNR.

FIG. 10 is a graphical illustration comparing the BER of precoded andnonprecoded artificial signal transmission for codebook-based MIMOsystems to other solutions known in the art as a function of SNR whereinthe correlation between the precoder/combiner and wireless transmissionchannels is 30%.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings, which form a part thereof, and withinwhich are shown by way of illustration specific embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural changes may be made withoutdeparting from the scope of the invention.

MIMO transmitters are known that perform precoding prior to transmissionto a MIMO receiver. The MIMO transmitter may estimate the transmissionchannel and then select one precoding/combining matrix pair from anumber of predefined precoding/combining matrix pairs provided by acodebook, based upon the estimate of the transmission channel. Theprecoding/combining matrices are unitary, and the precoding/combiningmatrix pair selected is the pair which will maximize the capacity basedon the estimate of the transmission channel. The MIMO transmitterprovides the selected precoding/combining matrix pair identifier to theMIMO receiver. The MIMO transmitter then applies the selected precodingmatrix to an information signal prior to transmission of the signal overthe transmission channel to the receiving antennae.

In various embodiments, the present invention utilizes a set ofMultiple-Input Multiple-Output (MIMO) transmitter precoding (andcorresponding receiver combining) matrices with finite cardinality.Knowing the exact channel coefficients between each transmitter andreceiver antenna at the MIMO transmitter, the method of the presentinvention calculates the artificial signal that minimizes the Euclideandistance between the desired and received processed data symbols. Insome embodiments, the artificial signal is then fed to the precoder,instead of the actual desired data symbols. In some embodiments, theartificial signal is fed directly to the MIMO transmitter's antennae,eliminating the need for precoding operation whenever possible.

The present invention addresses the issue of quantization error in MIMOsystems. In this context, quantization refers to quantization of aninfinite number of possible channel matrices to a finite set ofprecoding/combining matrices, and is not related to quantization ofanalog signals, as used widely in the electrical engineering literature.

In practical scenarios using analog/hybrid and digital codebook-basedprecoders and combiners, few samples are chosen from the infinitely manypossible V and corresponding U matrices. These samples are predefined asa dictionary at both ends of the transmission channel and only thesesample matrices are used at all times. Referring to the ith predefinedprecoder/combiner pair as V_(i) and U_(i), where i ∈ {1, 2, . . . , l};«∞. These precoders and combiners can then be realized using manymethods, including but not limited to, (1) Digital Signal Processing(DSP) programs, in the case of codebook based digital precoding, (2) DSPprograms with an RF switch, in the case of analog/digital hybridprecoding, (3) Phase shifters, in the case of analog precoding, (4)Forming special antenna array patterns, in the case of antenna arrayanalog precoding, and (5) Forming special antenna array patterns coupledwith optical lenses, in the case of lens aided antenna array precoding.

Because V_(i)≠V and U_(i)≠U, and having the practical post-processingoutput, there is an additional error term ∈:

{tilde over (y)}=U* _(i)UAV*V_(i) {tilde over (x)}

{tilde over (y)}=Λ{tilde over (x)}+∈

∈=U* _(i)AΛV*V_(i) {tilde over (x)}−U*UΛV*V{tilde over (x)}

∈=(U* _(i) −U*)UΛV*(V _(i) −V){tilde over (x)}

Where {tilde over (x)} is the information symbols, {tilde over (y)} isthe post-processing output, ∈ is the error due to the mismatch betweenprecoder/combiner matrix pair and the actual SVD components of thechannel. Throughout this document, U and V will be used to refer to theactual components of the SVD of the channel, and U_(i) and V_(i) will beused to refer to the predefined precoders and combiners that are usedduring the transmission.

In a first embodiment, adding artificial signals to data symbols thatforces the error to be zero is examined. In this embodiment, adding anartificial signal n to the information symbols {tilde over (x)}, priorthe precoder, is proposed, such that ∈ is reduced, i.e.

x=V _(i)({tilde over (x)}+n)

In this first embodiment, n=n_(ZF) where

U* _(i) UΛV*V _(i) n _(ZF)=−∈

U* _(i) UΛV*V _(i) n _(ZF)=−(U* _(i) −U*)UΛV*(V _(i) −V){tilde over (x)}

n _(ZF)=−(U* _(i) HV _(i))⁻¹(U* _(i) −U*)H(V _(i) −V){tilde over (x)}

However, as in all zero forcing cases, this operation is not powerlimited and as such, may yield artificial signal vectors having powerthat is greater than the power of the signal itself, which cannot betransmitted.

In a second embodiment, adding power limited artificial signals to thedata symbols, wherein the power limited artificial signal is obtainedusing convex optimization, is considered.

In this second embodiment, to prevent the unlimited power case and limitthe transmitted power, one can also compute and add the artificialsignal that does not completely eliminate, but minimizes, the mean ofthe square of E, such that the power of the vector input to the precoderof the transmitter is unity. Then, the power is divided between theactual signal and the artificial signal. If the actual signal power islimited to 1−α and the artificial signal power is limited to α, then:

$n = {\arg \; {\min\limits_{\overset{ˇ}{n}}{{{U_{i}^{*}U\; \Lambda \; V^{*}{V_{i}^{*}\left\lbrack {{\sqrt{\left( {1 - \alpha} \right)}\overset{\sim}{x}} + {\sqrt{\alpha}\overset{ˇ}{n}}} \right\rbrack}} - {\Lambda \; \overset{\sim}{x}}}}_{2}}}$${{subject}\mspace{14mu} {to}\mspace{14mu} {\overset{ˇ}{n}}_{2}} \leq \sqrt{N}$

Where n is the artificial signal to be added to the ideal signal and Nis the number of transmitting antennae of the MIMO transmitter. In thisembodiment, the optimization is convex and is practically highlyfeasible.

However, the power of the artificial signal (α) also needs to beoptimized. Accordingly, two additional embodiments are proposed whereinonly the artificial signal is shaped and transmitted and the actual datasignals are not part of the output.

In this third and fourth embodiments, the signal fed to the precoder ofthe MIMO transmitter is solely ñ, that is, x=V_(i)ñ, where x is thesignal fed to the transmitter antennae. The artificial signal isdesigned to look exactly like the desired data symbols after theprecoding, channel and post-processing transformation. This way, theequations become easier to solve with fewer parameters involved.

As such, the third embodiment proposes transmitting artificial signalthat eliminates the error. Similar to the first embodiment, equality ofthe practical output and the ideal output is forced, namely:

U* _(i) UΛV*V ñ _(ZF) =U*UΛV*V{tilde over (x)}

ñ _(ZF)=(U* _(i) UΛV*V _(i))⁻¹ U*HV{tilde over (x)}

ñ _(ZF)=(U* _(i) HV _(i))⁻¹ Λ{tilde over (x)}

However, similar to the first embodiment, the power of ñ_(ZF) isunlimited in this case and this cannot be practically used. Taking thesame approach as in the second embodiment to limit the power, apractical case in the fourth embodiment is provided.

The previously described first, second and third embodiments can beimplemented to solve the problem as described. However, theseembodiments are considered to provide a solution that exhibits lowerperformance and higher complexity. The following fourth and fifthembodiments of the present invention provide a high performing solutionhaving a reduced complexity.

The fourth embodiment proposes transmitting power limited artificialsignals that minimizes the error. As such, in the present invention, theultimate solution is to design the artificial signal with powerconstraints such that the error is minimized at the receiver is designedas:

$\overset{\sim}{n} = {\arg \; {\min\limits_{\overset{ˇ}{n}}{{{U_{i}^{*}U\; \Lambda \; V*V_{i}\overset{ˇ}{n}} - {\Lambda \; \overset{\sim}{x}}}}_{2}}}$${{subject}\mspace{14mu} {to}\mspace{14mu} {\overset{ˇ}{n}}_{2}} \leq \sqrt{N}$

Where ñ is the vector of signals fed to the precoder so that thepower-limited signal yielding the minimum error to the ideal input atthe output is designed.

Furthermore, in case of full digital beamforming, if the precodingoperation is performed completely in software and not using a fixedhardware, the artificial signal is designed in the fifth embodiment as:

$x = {\arg \; {\min\limits_{\overset{ˇ}{n}}{{{U_{i}^{*}U\; \Lambda \; V*\overset{ˇ}{n}} - {\Lambda \; \overset{\sim}{x}}}}_{2}}}$${{subject}\mspace{14mu} {to}\mspace{14mu} {\overset{ˇ}{n}}_{2}} \leq \sqrt{N}$

Where x is the vector of signals fed to the antennae. Compared to thefourth embodiment, this embodiment of the present invention reducescomputational complexity, power consumption and processing delay withoutany change in performance. If precoding is applied using a fixedhardware, the fourth embodiment may still be used by applying only asoftware upgrade to devices that are already produced and in use.

FIG. 1A illustrates a an ideal precoded/decoded MIMO block diagram 100in accordance with an embodiment of the present invention, whereinprecoding of the artificial signal is performed prior to transmission ofthe artificial signal over the transmission channel. Wherein, {tildeover (x)} 105 is the actual information symbols to be transmitted to theMIMO receiver and ñ 107 is the artificial signal generated by the MIMOtransmitter according to the method as described in the fourthembodiment of the present invention. The convex optimization module 110is used to generate the artificial signal ñ 107. V_(i) 112 is thepre-processing matrix, which is the precoding matrix. x 115 is thevector of signals fed to the transmitter antennae to be transmitted overthe transmission channel 120. y 125 is the column of observations at thereceiver antenna following transmission over the transmission channel120. U_(i)* 130 is the post-processing matrix based on the channel and{tilde over (y)} 135 is the estimated data symbols that are obtained bypost-processing the observations ({tilde over (y)}=Λ{tilde over (x)}).

FIG. 1B illustrates a precoded/decoded MIMO block diagram 150 inaccordance with a fifth embodiment of the present invention, wherein noprecoding of the artificial signal is performed prior to transmission ofthe artificial signal over the transmission channel. Wherein, {tildeover (x)} 155 is the actual information symbols to be transmitted to theMIMO receiver by the MIMO transmitter. The convex optimization module160 is used to generate the artificial signal x 165 according to thefifth embodiment of the present invention which is then fed to thetransmitter antennae to be transmitted over the transmission channel170. y 175 is the column of observations at the receiver antennafollowing transmission over the transmission channel 170. U_(i) 180 isthe post-processing matrix and ){tilde over (y)} 185 is the estimateddata symbols that are obtained by post-processing the observations({tilde over (y)}=Λ{tilde over (x)}).

FIG. 2 illustrates a pair of Multiple-Input Multiple-Output (MIMO)devices implementing the artificial signal scheme, according to oneembodiment of the present invention. As illustrated, a first MIMO device200 includes a MIMO transmitter 205, a MIMO receiver 210 and a number ofantennae 230, 235. The antennae 230, 235 are shared by the MIMOtransmitter 205 and the MIMO receiver 210. A second MIMO device 215 alsoincludes a MIMO transmitter 220 and a MIMO receiver 225, which shareantennae 240, 245.

In operation, the MIMO transmitter 205 of the MIMO device 200 appliesprecoding based on feedback received by the MIMO receiver 210 of theMIMO device 200 from the MIMO transmitter 220 of the MIMO device 215. Aspreviously described, in the present invention, the MIMO transmitter 205generates an artificial signal by performing convex optimization,wherein the artificial signal is designed to match desired data symbolsafter applying the combining matrix to the plurality of artificialsignals received over the wireless transmission channel. Depending uponthe embodiment being implemented, precoding may then be applied to thisartificial signal which is then transmitted over the transmittingantennae 230, 235 of the first MIMO device 200 to the receiving antennae240, 245 of the second MIMO device 215, for subsequent reception by theMIMO receiver 225.

FIG. 3 illustrates a flow diagram of the method operating acodebook-based multiple-input multiple-output (MIMO) transmitter, inaccordance with the present invention and current standard communicationprotocols. In this implementation, the precoding/combining 180 matrixpair is selected by the MIMO transmitter 200 and no precoding of theartificial signal 165 is performed prior to transmission of theartificial signal to the MIMO receiver 215. The method includes thesteps of, transmitting a known signal from a MIMO transmitter of theMIMO receiver 220 to a MIMO receiver of a MIMO transmitter 210 fortransmission channel 170 estimation 300 and estimating the transmissionchannel 170 at the MIMO transmitter 200 using the known signal from theMIMO receiver 305. The method further includes, selecting aprecoding/combining matrix pair at the MIMO transmitter 200 that bestmatches the transmission channel 170 estimation 310. The methodcontinues at step 315, wherein the MIMO transmitter 200 generates 160 anartificial signal 165 from an information signal 155 to be transmittedby the MIMO transmitter 200, wherein the artificial signal 165 minimizesan error between the information signal 155 to be transmitted to theMIMO receiver 215 over the transmission channel 170 and the informationsignal recovered 185 by the MIMO receiver 215 following the applicationof a combining operation 180 based upon the precoding/combining 180matrix pair. The method continues by transmitting the artificial signal165 and the precoding/combining matrix pair identifier to the MIMOreceiver 215 over the transmission channel 320. The MIMO receiver 215then applies the combining operation 180 to the received artificialsignal 175 based upon the precoding/combining 180 matrix pair to recoverthe information signal 325.

In an additional embodiment of the present invention, illustrated withreference to FIG. 4, the precoding 112/combining 130 matrix pair isselected by the MIMO transmitter 200 and precoding 112 of the artificialsignal 107 is performed prior to transmission of the artificial signal107 to the MIMO receiver 215. The method includes the steps of,transmitting a known signal from a MIMO transmitter of the MIMO receiver220 to a MIMO receiver of the MIMO transmitter 210 for transmissionchannel 120 estimation 400 and estimating the transmission channel 120at the MIMO transmitter 200 using the known signal from the MIMOreceiver 405. The method further includes, selecting a precoding112/combining 130 matrix pair at the MIMO transmitter 200 that bestmatches the transmission channel 120 estimation 410. The methodcontinues at step 415, wherein the MIMO transmitter 200 generates 110 anartificial signal 107 from an information signal to be transmitted 105by the MIMO transmitter 200, wherein the artificial signal 107 minimizesan error between the information signal 105 to be transmitted to theMIMO receiver 215 over the transmission channel 120 and the informationsignal recovered 135 by the MIMO receiver 215 following the applicationof a combining operation 130 based upon the precoding 112/combining 130matrix pair. The method continues at step 420, wherein the MIMOtransmitter 200 performs precoding 112 of the artificial signal 107prior to transmitting the precoded artificial signal 115 and theprecoding 112/combining 130 matrix pair identifier to the MIMO receiver215 over the transmission channel 425. The MIMO receiver 215 thenapplies the combining operation 130 to the precoded artificial signalreceived over the wireless transmission channel 125 based upon theprecoding 112/combining 130 matrix pair to recover the informationsignal 430.

In another embodiment of the present invention, illustrated withreference to FIG. 5, the precoding/combining matrix is selected by theMIMO receiver 215 and no precoding of the artificial signal 165 isperformed prior to transmission of the artificial signal to the MIMOreceiver 215. The method includes the steps of, transmitting a knownsignal from a MIMO transmitter of a MIMO transmitter 205 to a MIMOreceiver of a MIMO receiver 225 for transmission channel 170 estimation500 and estimating the transmission channel 170 at the MIMO receiver 215using the known signal from the MIMO receiver 505. The method furtherincludes, selecting a precoding/combining 180 matrix pair at the MIMOreceiver 215 that best matches the transmission channel 170 estimation510. The method continues at step 515, wherein the MIMO transmitter ofthe MIMO receiver 220 transmits the precoding/combining 180 matrix pairidentifier and the known signal to the MIMO receiver of the MIMOtransmitter 210. The MIMO transmitter then estimates the transmissionchannel 170 using the known signal received from the MIMO receiver 520.The method continues at step 525, wherein the MIMO transmitter 200generates 160 an artificial signal 165 from an information signal 155 tobe transmitted by the MIMO transmitter 200, wherein the artificialsignal 165 minimizes an error between the information signal 155 to betransmitted to the MIMO receiver 215 over the transmission channel 170and the information signal recovered by the MIMO receiver 185 followingthe application of a combining operation 180 based upon theprecoding/combining 180 matrix pair. The method continues at step 530,wherein the MIMO transmitter of the MIMO transmitter 210 transmits theartificial signal 165 to the MIMO receiver of the MIMO receiver 225 overthe transmission channel 170. The MIMO receiver 215 then applies thecombining operation 180 to the received artificial signal 175 based uponthe precoding/combining 180 matrix pair to recover the informationsignal 535.

In an additional embodiment of the present invention, illustrated withreference to FIG. 6, the precoding 112/combining 130 matrix is selectedby the MIMO receiver 215 and precoding 112 of the artificial signal 107is performed prior to transmission of the precoded artificial signal 115to the MIMO receiver 215. The method includes the steps of, transmittinga known signal from a MIMO transmitter of a MIMO transmitter 205 to aMIMO receiver of a MIMO receiver 225 for transmission channel 120estimation 600 and estimating the transmission channel 120 at the MIMOreceiver 215 using the known signal from the MIMO transmitter 605. Themethod further includes, selecting a precoding 112/combining 130 matrixpair at the MIMO receiver 215 that best matches the transmission channel120 estimation 610. The method continues at step 615, wherein a MIMOtransmitter of the MIMO receiver 220 transmits the precoding112/combining 130 matrix pair identifier and the known signal to a MIMOreceiver of the MIMO transmitter 210. The MIMO transmitter 200 thenestimates the transmission channel 120 using the known signal receivedfrom the MIMO receiver 620. The method continues at step 625, whereinthe MIMO transmitter 200 generates 110 an artificial signal 107 from aninformation signal 105 to be transmitted by the MIMO transmitter 200,wherein the artificial signal 107 minimizes an error between theinformation signal 105 to be transmitted to the MIMO receiver 215 overthe transmission channel 120 after the precoding 112 to obtain theprecoded artificial signal 115 and the information signal recovered bythe MIMO receiver 135 following the application of a combining operation130 based upon the precoding 112/combining 130 matrix pair. The methodcontinues at step 630, wherein the MIMO transmitter 200 precodes 112 theartificial signal 107 prior to transmitting the precoded artificialsignal 115 to the MIMO receiver 215 over the transmission channel 635.The MIMO receiver 215 then applies the combining operation 130 to thereceived precoded artificial signal 125 based upon the precoding112/combining 130 matrix pair to recover the information signal 640.

The present invention provides several benefits, including increasedcapacity, increased secrecy while requiring no changes at the receiver.Increased capacity is provided because the mutual information betweenthe received data using the proposed system is higher than the legacysystems as the symbols are more similar, thereby increasing capacity.Increased secrecy is provided as conventional schemes yield a finite setof possible precoder outputs as the set of information symbols and theset of precoding matrices are finite, whereas in this case, infinitelymany different transmission symbols may be generated.

FIG. 7 is a graphical illustration of the increase in capacity andsecrecy provided by the artificial signal transmission in codebook-basedMIMO systems in accordance with the present invention. FIG. 7 shows theerror magnitude for infinite SNR as it is relates to the similaritybetween the physical channel between the MIMO transmitter and intendedMIMO receiver and the channel induced by practical precoder/combiner; atthe desired MIMO receiver having 4 antennae if; the MIMO transmitterhaving 8 antennae transmits data in the conventional manner 700, theMIMO transmitter having 16 antennae transmits data in the conventionalmanner 705, the MIMO transmitter having 8 antennae transmits data usingthe proposed precoded artificial signal transmission method 710, theMIMO transmitter having 16 antennae transmits data using the proposedprecoded artificial signal transmission method 715, the MIMO transmitterhaving 8 antennae transmits data using the proposed nonprecodedartificial signal transmission method 720, the MIMO transmitter having16 antennae transmits data using the proposed nonprecoded artificialsignal transmission method 730; and at a MIMO eavesdropper having 32antennae if; the MIMO transmitter having 8 antennae transmits data usingthe proposed precoded artificial signal transmission method 735, theMIMO transmitter having 16 antennae transmits data using the proposedprecoded artificial signal transmission method 740. As illustrated, theerror at the desired receiver decreases as a result of the use of theinventive artificial signal transmission methods proposed in thisapplication which increases capacity of the channel, whereas the errorat an eavesdropper increases as a result of the use of the inventiveartificial signal which increases the secrecy of the channel.

The predefined precoder and combiners are commonly known. If pure datain the standard form is transmitted, anyone can intercept the pure dataand decode it by trying to decode it using all of the combiners definedin the standard. If artificial signal that is defined based on thechannel of the intended receiver is transmitted instead of standarddata, an eavesdropper of which experienced channel is different fromthat of the intended receiver will see noise after the standardcombination process. This provides physical security, especially foreavesdroppers that are geographically far from the intended receiver.

FIG. 8 is a graphical illustration comparing the secrecy provided by theartificial signal transmission in codebook-based MIMO systems inaccordance with the present invention. FIG. 8 shows the secrecy capacitybetween the desired MIMO receiver having 4 antennae and the MIMOeavesdropper having 32 antennae for infinite SNR as it is relates to thesimilarity between the physical channel between the MIMO transmitter andintended MIMO receiver and the channel induced by practicalprecoder/combiner if; the MIMO transmitter having 8 antennae transmitsdata using the proposed precoded artificial signal transmission method800, the MIMO transmitter having 16 antennae transmits data using theproposed precoded artificial signal transmission method 805, the MIMOtransmitter having 8 antennae transmits data using the proposednonprecoded artificial signal transmission method 810, the MIMOtransmitter having 16 antennae transmits data using the proposednonprecoded artificial signal transmission method 820. In accord withFIG. 7, FIG. 8 also illustrates that the secrecy capacity dependsheavily on number of MIMO transmitter antennae, proposed nonprecodedartificial signal is more secure than precoded artificial signal asthere is more flexibility to the signal design. The secrecy capacity ofprior art for the given number of antennae is zero hence is not shown.

FIG. 9 is a graphical illustration of the increase in capacity andsecrecy provided by the artificial signal transmission in codebook-basedMIMO systems in accordance with the present invention. FIG. 9 shows thebit-error-rate (BER) for 3 decibel SNR as it is relates to thesimilarity between the physical channel between the MIMO transmitter andintended MIMO receiver and the channel induced by practicalprecoder/combiner; at the desired MIMO receiver having 4 antennae if;the MIMO transmitter having 8 antennae transmits data in theconventional manner 900, the MIMO transmitter having 16 antennaetransmits data in the conventional manner 905, the MIMO transmitterhaving 8 antennae transmits data using the proposed precoded artificialsignal transmission method 910, the MIMO transmitter having 16 antennaetransmits data using the proposed precoded artificial signaltransmission method 915, the MIMO transmitter having 8 antennaetransmits data using the proposed nonprecoded artificial signaltransmission method 920, the MIMO transmitter having 16 antennaetransmits data using the proposed nonprecoded artificial signaltransmission method 925; and at a MIMO eavesdropper having 32 antennaeif the MIMO transmitter having 8 antennae transmits data using theproposed precoded artificial signal transmission method 930. Asillustrated, although the number of MIMO transmitter antennae causes asignificant increase in all cases, even the proposed precoded artificialsignal transmission scheme at least halves the BER of prior art usinghalf the number of transmitter antennae, whereas the gain of proposednonprecoded artificial signal transmission is always of greatermagnitude. The gain of proposed precoded artificial signal transmissionscheme diminishes with increasing channel correlation whereas the gainof proposed nonprecoded artificial signal transmission scheme continuesgrowing linearly in decibel scale without bottlenecking. The performanceat eavesdropper remains flat for both proposed methods (the nonprecodedartificial signal transmission scheme is not shown for clarity as itoverlaps), and remains close to the performance of desired receiver forconventional transmission.

FIG. 10 is a graphical illustration of the increase in capacity andsecrecy provided by the artificial signal transmission in codebook-basedMIMO systems in accordance with the present invention. FIG. 10 shows thebit-error-rate (BER) for the similarity between the physical channelbetween the MIMO transmitter and intended MIMO receiver and the channelinduced by practical precoder/combiner being equal to 30% as it isrelates to SNR; at the desired MIMO receiver having 4 antennae if; theMIMO transmitter having 8 antennae transmits data in the conventionalmanner 1000, the MIMO transmitter having 16 antennae transmits data inthe conventional manner 1005, the MIMO transmitter having 8 antennaetransmits data using the proposed precoded artificial signaltransmission method 1010, the MIMO transmitter having 16 antennaetransmits data using the proposed precoded artificial signaltransmission method 1015, the MIMO transmitter having 8 antennaetransmits data using the proposed nonprecoded artificial signaltransmission method 1020; and at a MIMO eavesdropper having 32 antennaeif; the MIMO transmitter having 8 antennae transmits data using theproposed precoded artificial signal transmission method 1025, and theMIMO transmitter having 8 antennae transmits data using the proposednonprecoded artificial signal transmission method 1030. As illustrated,the BER is independent of SNR for the proposed nonprecoded artificialsignal transmission scheme, but depends on the number of transmitterantennae as expected (BER for nonprecoded artificial signal transmissionscheme from MIMO transmitter having 16 antennae is similarly constant ata lesser level and is not shown to maintain resolution at this level).The precoded artificial signal transmission scheme converges to the BERas the nonprecoded artificial signal transmission scheme at 6 dB SNR,whereas the prior art converges to the same performance at 10 dB SNR,independent of the number of transmitter antennae. The BER at theeavesdropper is independent of whether the proposed precoded ornonprecoded artificial signal transmission scheme is used, hence whilethe security gain is theoretically dependent on which of the proposedschemes is utilized for high SNR values, is independent of the scheme inlower, realistic SNR values.

Because the invention is inherently designed to exploit theimperfections at the receiver, this invention does not require anychanges to the receiving device. The receiving device still receives thesignal from the predefined combiners, as conventional in the art. Theimplementation of the invention is at the transmitter only, therebyrequiring no modification to the receiver and no changes to thestandards. The inventive concept can be implemented at any transmitterthat desires to exploit this invention, even by devices that aredesigned to communicate using standards that are complete.

The primary contribution of the present invention is the formulation ofthe convex optimization problem with the predeterminedprecoder/combiners and the actual channel of the desired user. In thepresent invention, convex formulation allows fast signal design with lowpower consumption and computational complexity. Additionally, digitalformulation makes the invention independent of the hardware platform anduniversally usable for all hardware configurations. While other knownartificial signal (at times referred to as noise) generation algorithmsfor MIMO communication systems usually separate the desired symbols fromthe design and use artificial signal to enhance the secrecy of thesystem, whereby two separate signals are added together and transmittedwith different powers, to address two contradicting goals. In contrast,in accordance with the convex optimization solution provided by thepresent invention, both goals are achieved using a single solution, anda single signal design achieves both goals, simultaneously.

Hardware and Software Infrastructure Examples

The present invention may be embodied on various computing platformsthat perform actions responsive to software-based instructions and mostparticularly on touchscreen portable devices. The following provides anantecedent basis for the information technology that may be utilized toenable the invention.

The computer readable medium described in the claims below may be acomputer readable signal medium or a computer readable storage medium. Acomputer readable storage medium may be, for example, but not limitedto, an electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing. More specific examples (a non-exhaustive list) of thecomputer readable storage medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any non-transitory, tangiblemedium that can contain, or store a program for use by or in connectionwith an instruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device. However, asindicated above, due to circuit statutory subject matter restrictions,claims to this invention as a software product are those embodied in anon-transitory software medium such as a computer hard drive, flash-RAM,optical disk or the like.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wire-line, optical fiber cable, radio frequency, etc., or any suitablecombination of the foregoing. Computer program code for carrying outoperations for aspects of the present invention may be written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, C#, C++, Visual Basic or thelike and conventional procedural programming languages, such as the “C”programming language or similar programming languages.

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

It should be noted that when referenced, an “end-user” is an operator ofthe software as opposed to a developer or author who modifies theunderlying source code of the software. For security purposes,authentication means identifying the particular user while authorizationdefines what procedures and functions that user is permitted to execute.

The advantages set forth above, and those made apparent from theforegoing description, are efficiently attained. Since certain changesmay be made in the above construction without departing from the scopeof the invention, it is intended that all matters contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

What is claimed:
 1. A codebook-based multiple-input multiple-output(MIMO) transmission method, the method comprising: selecting aprecoding/combining matrix pair, wherein the precoding/combining matrixpair is selected based upon an estimated channel coefficient of atransmission channel between a MIMO transmitter and a MIMO receiver;receiving an information signal at the MIMO transmitter; and generatingan artificial signal from the information signal, wherein the artificialsignal minimizes an error between the information signal and a signalrecovered by the MIMO receiver following application of the combiningmatrix of the precoding/combining matrix pair to the artificial signalreceived over the transmission channel.
 2. The method of claim 1,further comprising: transmitting the artificial signal to the MIMOreceiver over the transmission channel; and applying the combiningmatrix to the received artificial signal based upon theprecoding/combining matrix pair at the MIMO receiver to recover theinformation signal.
 3. The method of claim 2, further comprising,applying a precoding operation to the artificial signal based upon theprecoding/combining matrix pair prior to transmitting the precodedartificial signal to the MIMO receiver, wherein the artificial signalminimizes an error between the information signal and a signal recoveredby the MIMO receiver following application of the combining matrix ofthe precoding/combining matrix pair to the artificial signal receivedover the transmission channel and following the application of theprecoding matrix of the precoding/combining matrix pair.
 4. The methodof claim 1, further comprising: transmitting a known signal from theMIMO receiver to the MIMO transmitter; estimating the channelcoefficient at the MIMO transmitter based upon the known signal;selecting the precoding/combining matrix pair at the MIMO transmitter;and transmitting the selected precoding/combining matrix pair identifierto the MIMO receiver.
 5. The method of claim 1, further comprising:transmitting a known signal from the MIMO transmitter to the MIMOreceiver; estimating the channel coefficient at the MIMO receiver basedupon the known signal; selecting the precoding/combining matrix pair atthe MIMO receiver; transmitting the precoding/combining matrix pairidentifier and the known signal to the MIMO transmitter; and estimatingthe channel coefficient at the MIMO transmitter based upon the knownsignal
 6. The method of claim 1, wherein the estimated channelcoefficient of the transmission channel is a matrix comprising anestimated channel coefficient between each transmitter and receiverantenna pair at the MIMO transmitter
 7. The method of claim 1, whereingenerating an artificial signal further comprises performing convexoptimization to generate the artificial signal.
 8. A codebook-basedmultiple-input multiple-output (MIMO) method, the method comprising:selecting a precoding/combining matrix pair, wherein theprecoding/combining matrix pair is selected based upon an estimatedchannel coefficient of a transmission channel between a MIMO transmitterand a MIMO receiver; receiving an information signal at the MIMOtransmitter; generating an artificial signal from the informationsignal, wherein the artificial signal minimizes an error between theinformation signal and a signal recovered by the MIMO receiver followingapplication of the combining matrix of the precoding/combining matrixpair to the artificial signal over the transmission channel;transmitting the artificial signal to the MIMO receiver over thetransmission channel; and applying the combining matrix to the receivedartificial signal based upon the precoding/combining matrix pair at theMIMO receiver to recover the information signal.
 9. The method of claim8, wherein generating an artificial signal further comprises performingconvex optimization to generate the artificial signal.
 10. A codebookbased multiple input multiple output (MIMO) transmitter, the transmittercomprising: a signal processing unit for receiving a precoding/combiningmatrix pair identifier and for receiving an information signal, whereinthe precoding/combining matrix pair identifier is based upon anestimated channel coefficient of a transmission channel between a MIMOtransmitter and a MIMO receiver; and the signal processing unit furtherfor generating an artificial signal from the information signal, whereinthe artificial signal minimizes an error between the information signaland the signal recovered by the MIMO receiver following application ofthe combining matrix of the precoding/combining matrix pair to theartificial signal received over the transmission channel.
 11. The MIMOtransmitter of claim 10, wherein the signal processing unit is furtherconfigured for transmitting the artificial signal to the MIMO receiverover the transmission channel for the application of the combiningmatrix to the artificial signal based upon the precoding/combiningmatrix pair at the MIMO receiver to recover the information signal. 12.The MIMO transmitter of claim 11, wherein the signal processing unit isfurther configured for applying a precoding operation to the artificialsignal based upon the precoding/combining matrix pair prior totransmitting the precoded artificial signal to the MIMO receiver,wherein the artificial signal minimizes an error between the informationsignal and a signal recovered by the MIMO receiver following applicationof the combining matrix of the precoding/combining matrix pair to theartificial signal received over the transmission channel and followingthe application of the precoding matrix of the precoding/combiningmatrix pair.
 13. The MIMO transmitter of claim 10, wherein the signalprocessing unit is further configured for: receiving a known signal froma MIMO receiver over the transmission channel; estimating the channelcoefficient based upon the known signal; selecting theprecoding/combining matrix pair; and transmitting the selectedprecoding/combining matrix pair identifier to the MIMO receiver.
 14. TheMIMO transmitter of claim 10, wherein the signal processing unit isfurther configured for: transmitting a known signal to the MIMO receiverover the transmission channel; receiving the precoding/combining matrixpair identifier from the MIMO receiver based upon the known signal; andestimating the channel coefficient based upon the known signal.
 15. TheMIMO transmitter of claim 10, wherein the estimated channel coefficientof the transmission channel is a matrix comprising an estimated channelcoefficient between each transmitter and receiver antenna pair at theMIMO transmitter.
 16. The MIMO transmitter of claim 10, whereingenerating an artificial signal further comprises performing convexoptimization to generate the artificial signal.
 17. The MIMO transmitterof claim 10, wherein the signal processing unit is a modem.
 18. One ormore non-transitory computer-readable media having computer-executableinstructions for performing a method of running a software program on acomputing device, the computing device operating under an operatingsystem, the method including issuing instructions from the softwareprogram comprising: selecting a precoding/combining matrix pair, whereinthe precoding/combining matrix pair is selected based upon an estimatedchannel coefficient of a transmission channel between a MIMO transmitterand a MIMO receiver; receiving an information signal at the MIMOtransmitter; generating an artificial signal from the informationsignal, wherein the artificial signal minimizes an error between theinformation signal and a signal recovered by the MIMO receiver followingapplication of the combining matrix of the precoding/combining matrixpair to the artificial signal received over the transmission channel.19. The non-transitory computer-readable media of claim 18, wherein theestimated channel coefficient of the transmission channel is a matrixcomprising an estimated channel coefficient between each transmitter andreceiver antenna pair at the MIMO transmitter.
 20. The non-transitorycomputer-readable media of claim 18, wherein generating an artificialsignal further comprises performing convex optimization to generate theartificial signal.